The present invention relates to a transfer device that transfers a toner image from a plurality of image bearing members to paper through an intermediate transfer belt.
Among the image forming apparatus adopting the electrophotography method is one equipped with a transfer device that is based on the intermediate transfer method in which toner images are transferred in such a manner as to be superimposed sequentially from a plurality of image bearing members onto an intermediate transfer belt in the primary transfer and then the toner image is transferred from the intermediate transfer belt onto paper in the secondary transfer.
In the transfer device that is based on the intermediate transfer method and is installed in a color image forming apparatus, the intermediate transfer belt is caused to be displaced depending on its operational modes, that is to say, monochromatic image forming mode, color image forming mode and standby mode. The intermediate transfer belt comes into contact with only an image bearing member for black in the monochromatic image forming mode, comes into contact with all the image bearing members in the color image forming mode, and is separate from all the image bearing members in the standby mode.
For example, as disclosed in Japanese Patent Unexamined Publication No. 2010-134149 bulletin, transition between the intermediate transfer belt's contact with and separation from the image bearing members is achieved by causing the plurality of primary transfer rollers opposed to the plurality of image bearing members across the intermediate transfer belt to be displaced.
The conventional transfer device as described in the Patent Document above includes a link member for black to cause the primary transfer roller for black to be displaced, a link member for color to cause the primary transfer rollers for color to be displaced, cams to cause the link member for black and the link member for color to respectively move and a drive source to rotate the cams. A longitudinal movement of the link member for black causes the primary transfer roller for black to be displaced toward contact-separation directions in relation to the image bearing member for black, and a longitudinal movement of the link members for color causes the primary transfer rollers for color to be displaced toward contact-separation directions in relation to the image bearing members for color.
Even when a resin material such as POM (polyoxymethylene) resin or the like is used for the link member for black for reasons such as improved sliding property with the cam and prevention of electric leakage etc., a thermally induced dimensional change of the link member for black is small because the link member for black is shorter as compared with the link member for color. As a result, a displacement error due to a temperature change is relatively small in the case of the primary transfer roller for black; therefore, movement of the intermediate transfer belt's contact with and separation from the image bearing member is operable rather steadily.
Whereas, the thermally induced dimensional change of the link member for color amounts to big because it is longer as compared with the link member for black so as to cause the plurality of primary transfer rollers to be displaced. For example, with a 240 mm long link member for color formed using the POM resin, when a temperature range of 5-45 degrees Celsius as an internal temperature of the image forming apparatus and an occurrence of a temperature change of 10 degrees Celsius are assumed, a thermally induced dimensional change of +−0.3 mm is expected to occur. If a temperature change of 30 degrees Celsius occurs, the thermally induced dimensional change amounts to an extent as big as 1 mm.
Accordingly, accuracy in an arrangement of the primary transfer rollers for color that are linked to the movement of the link member for color deteriorates due to the change of the internal temperature of the image forming apparatus. As a result, the movement of the intermediate transfer belt's contact with and separation from the image bearing members for color becomes unsteady; so that picture quality is prone to deteriorate. Among the primary transfer rollers for color, a primary transfer roller joined to the other end portion on the opposite side of an end portion on the side that is in contact with the cam of the link member for color with pressure is most subject to harmful effect resulting from the thermally induced dimensional change of the link member for color, so that accuracy in its arrangement is more liable to deteriorate.
For example, because a contact pressure of an intermediate transfer belt against the image bearing members for color becomes too strong when the link member for color elongates due to the thermally induced dimensional change, problems such as deterioration of picture quality and shortening of life cycle due to abrasion of the image bearing members for color and the intermediate transfer belt can occur.
Further, when the link member for color becomes shortened due to its thermal shrinkage, there arises a risk that the intermediate transfer belt will not come into contact with the image bearing members for color; so that the problem of picture quality deterioration can occur.
The present invention is directed to providing a transfer device capable of suppressing the deterioration of accuracy in an arrangement of primary transfer rollers due to a thermally induced dimensional change of a link member.